Molten salt synthesis of disordered spinel CoFe2O4 with improved electrochemical performance for sodium-ion batteries

Sodium-ion (Na-ion) batteries are currently being investigated as an attractive substitute for lithium-ion (Li-ion) batteries in large energy storage systems because of the more abundant and less expensive supply of Na than Li. However, the reversible capacity of Na-ions is limited because Na possesses a large ionic radius and has a higher standard electrode potential than that of Li, making it challenging to obtain electrode materials that are capable of storing large quantities of Na-ions. This study investigates the potential of CoFe2O4 synthesised via the molten salt method as an anode for Na-ion batteries. The obtained phase structure, morphology and charge and discharge properties of CoFe2O4 are thoroughly assessed. The synthesised CoFe2O4 has an octahedron morphology, with a particle size in the range of 1.1–3.6 μm and a crystallite size of ∼26 nm. Moreover, the CoFe2O4 (M800) electrodes can deliver a high discharge capacity of 839 mA h g−1 in the first cycle at a current density of 0.1 A g−1, reasonable cyclability of 98 mA h g−1 after 100 cycles and coulombic efficiency of ∼99%. The improved electrochemical performances of CoFe2O4 can be due to Na-ion-pathway shortening, wherein the homogeneity and small size of CoFe2O4 particles may enhance the Na-ion transportation. Therefore, this simple synthetic approach using molten salt favours the Na-ion diffusion and electron transport to a great extent and maximises the utilisation of CoFe2O4 as a potential anode material for Na-ion batteries.


Introduction
7][8] Additionally, low energy density is another drawback of Na-ions, because sodium is heavier (23 g mol −1 ) and has a lower redox potential (−2.71 V vs. standard hydrogen electrode (SHE)) compared to lithium (−3.02V vs. SHE). 9To address these issues, determining a suitable anode material is essential.Designing anodes with materials having a high specic capacity, high conductivity and high sodium storage capacity has proven to be a successful strategy for Naion batteries. 10,11Previously, various materials, [12][13][14] including transition metal oxides (TMOs) such as Mn 2 O 3 , 15 Mn 3 O 4 , 16 Fe 3 O 4 17 and Co 3 O 4 18 have been investigated for synthesising anodes for Na-ion batteries.However, TMOs possess low electrical conductivity and exhibit large volume change because the active material particles swell and shrink in response to the insertion and extraction of Na-ions during charging and discharging. 19ecently, various studies have focused on iron-based (Febased) oxide anode materials [20][21][22] and spinel ferrites, with the formula AFe 2 O 4 (A = Mn, Co, Cu and Ni), are considered to display greater performance than simple iron oxide and to have the advantages of natural abundance, non-toxicity and cost efficiency. 23,246][27] Based on a previous report, 10 MgFe 2 O 4 was synthesised using a microwave-assisted method, demonstrating outstanding electrochemical performance and good cyclability.In MgFe 2 O 4 , spinel ferrite acts as a buffer for the matrix to maintain structural stability and reduce the effect of volume change during charging and discharging. 28,29Additionally, the spinel ferrite performs better than single oxide and has higher electrical conductivity. 30,31nterestingly, cobalt ferrite (CoFe 2 O 4 ) has attracted research attention as a potential anode material for Na-ion batteries.CoFe 2 O 4 consists of two metal ions capable of accepting multiple electrons, demonstrating superior performance with a high theoretical capacity of 916 mA h g −1 . 32Zhang et al. 33 reported the porous CoFe 2 O 4 nanocubes delivered a high capacity of 360 mA h g −1 aer 50 cycles and displayed a high initial coulombic efficiency of 68.8%.In another work, He et al. 23 synthesised CoFe 2 O 4 through a hydrothermal technique.In the rst cycle, the discharge capacity of the CoFe 2 O 4 was 300 mA h g −1 (current density of 100 mA g −1 ); however, the capacity faded rapidly.Similarly, Feng et al. 34 synthesised CoFe 2 O 4 via hydrothermal method and demonstrated a discharge capacity of approximately 200 mA h g −1 (at a current density of 0.05 A g −1 ) aer 90 cycles.Hence, further research needs to be conducted to enhance the electrochemical performance of CoFe 2 O 4 -based anodes, which can be accomplished by exploring synthesis different synthesis methods because synthesis methods can impact electrochemical performances.
To date, different methods have been developed for synthesising CoFe 2 O 4 , including hydrothermal, 35 mechanical-alloying 36 and ball-milling 37 methods.The synthesis methods can affect the structure, properties, morphologies, phase purity and crystallinity of CoFe 2 O 4 . 38The molten salt method may offer more advantages compared to other methods, such as well-dened facets despite reactions taking place at lower temperatures within a short time, highly homogeneous product formation and reduced particle agglomeration. 39,40Besides, numerous studies have been reported on the synthesis of CoFe 2 O 4 via the molten salt method with various salt combinations.Yang et al. 41 used Li 2 SO 4 /Na 2 SO 4 and NaCl/KCl to synthesise CoFe 2 O 4 using the molten salt method for the rst time.The CoFe 2 O 4 particles are well formed and many particles have octahedron shape, indicating that an interface reaction mechanism regulates particle growth.Another study demonstrated that CoFe 2 O 4 synthesised through the molten salt method using NaCl and KCl exhibited excellent electrochemical performance in Li-ion batteries with good cyclability and high reversible capacity. 42erein, we report that the CoFe 2 O 4 synthesised via the molten salt method using NaCl and KCl as precursors yields a remarkable electrochemical performance as an anode material in Na-ion batteries.During the synthesis, the molten salt helps control the particle size and shape at low temperatures and protects particles from agglomeration, resulting in homogeneous particles.The most important aspect of this structure is the octahedron shape of the CoFe 2 O 4 particle, which is between 1.1 and 3.6 mm in size and can provide sites for reaction with Na-ions. 43The unique structure of the octahedron CoFe 2 O 4 particle notably enhanced the electrochemical performance of CoFe 2 O 4 , with a high initial discharge capacity of 839 mA h g −1 and capacity retention of 98 mA h g −1 at 0.1 A g −1 aer 100 cycles, indicating the remarkable potential of CoFe 2 O 4 as an anode material.

Material characterisation
X-ray diffraction (XRD) pattern of the sample was obtained using Rigaku Miniex II under monochromatic Cu-Ka radiation (l = 1.5148Å) from 5°to 80°.The morphology and structure of the CoFe 2 O 4 were viewed using scanning electron microscopy (SEM; JEOL JSM-6360LA) and transmission electron microscope (TEM; TECNAI G2 F20).The Fourier-transform infrared (FTIR) spectra were obtained using Shimadzu IR Tracer-100.The X-ray photoelectron spectroscopy (XPS) was used to examine the chemical states of the elements present in the CoFe 2 O 4 using an Axis Ultra DLD XPS, Kratos and obtained spectra were tted using CASA soware.Then, Raman spectroscopy (Renishaw) was performed using 532 nm excitation extended with 0.1% power-laser measurements.

Electrochemical measurements
All chemicals were obtained from Sigma-Aldrich.The CoFe 2 O 4 electrode is prepared by mixing 75 wt% active material, 15 wt% carbon black and 10 wt% polyvinylidene uoride (PVDF) in Nmethyl-2-pyrrolidone (NMP).The slurry was applied onto a copper foil with an electrode mass loading of ∼2 mg cm −2 and dried at 100 °C overnight.The coin-type cell (CR2032) was assembled in an argon-lled glove box (MBRAUN Unilab) with sodium metal as the counter electrode and glass bre as the separator.The electrolyte solution was prepared using 1 M NaClO 4 (98%) in a mixture of propylene carbonate (anhydrous, 99.7%) with 5 wt% uoroethylene carbonates (99%).Cyclic voltammetry (CV; CHI 700E) and galvanostatic charge and discharge (NEWARE battery analyser) were controlled in a range of 0.01-3.00No additional peaks are observed, demonstrating the purity of the CoFe 2 O 4 produced.Overall, the intensity peaks become sharper as the calcination temperature increases, indicating that the crystallite size increases with temperature. 46The crystallite sizes (L) for all samples were calculated using Scherrer's equation:

Results and discussion
where, k is a constant (0.9394), l is the Cu-Ka radiation wavelength (1.5148 Å), b is the full width at half-maximum on the XRD peak in radians and q is the angle of diffraction.M700, M800 and M900 crystallite sizes were calculated to be 26.26,29.84 and 31.36 nm, respectively.Moreover, the lattice parameters, a for the calcination treatment for M700, M800 and M900 slightly decrease by 8.37, 8.36 and 8.33 Å, respectively.This phenomenon is typically occurs due to the defects removal such as oxygen vacancies and the lattice contracts during calcination. 47These ndings are similar to the results from a previous study. 48aman spectroscopy (Fig. 2) was also carried out to conrm the nature of CoFe 2 O 4 .Inverse spinel CoFe 2 O 4 shows an A 1g symmetry at 684 cm −1 associated with the tetrahedral sublattice and octahedral sub-lattice at the peak at 615 cm −1 . 49,50he band at 473 cm −1 is attributed to asymmetric bending of Fe (Co)-O. 50Conversely, the Raman band at 291 cm −1 is attributed to the E g symmetric bending of Fe (Co)-O. 51he formation of CoFe 2 O 4 spinel was also supported by the FTIR spectra (Fig. 3).The appearance of two peaks at 570 and 415 cm −1 is closely linked to the stretching vibrations of metal oxide in the octahedral site Co 2+ -O 2− and tetrahedral site Fe 3+ -O 2− , respectively. 52,53These two typical bands can be detected in almost all CoFe 2 O 4 structures. 54However, at relatively higher temperatures, the peaks become sharper and narrower due to lattice distortion minimization and improve the crystallinity. 55his fact is in agreement with XRD.
XPS spectroscopy was explained the elemental composition of CoFe 2 O 4 and displays the existence of the Co, Fe and O element as showed in Fig. 4. The deconvoluted spectra of the Co 2p (Fig. 4a) spectra show peaks due to Co 2p 3/2 and Co 2p 1/2 at binding energy of 779.71 and 794.99 eV respectively. 56In addition, the satellites peaks at 785.38 eV and 802.83 eV indicated the presence of unpaired 3d electron of the high spin Co 2+ . 57,58n Fig. 4b exposed Fe 2p spectrum and displayed the Fe 2p 3/2 and Fe 2p 1/2 peaks at 710.54 and 723.58 eV, respectively.These results support the presence of Fe 3+ in the inverse spinel CoFe 2 O 4 . 58The two peaks at 529.42 and 532.31 eV in a single O 1s ne spectra (Fig. 4c) can be considered as the metal-O bond and consistent with oxygen in the defect, respectively. 56,59EM images (Fig. 5) showed a remarkable morphological change as the calcination temperatures increased with average particle size ranging from 1.1 to 3.6 mm.Sample M700 (Fig. 5a) shows an octahedron shape with a particle size of ∼1.1 mm, and sample M800 (Fig. 5b) shows a well-dened octahedral shape, with a faceted surface and size of about ∼2.27 mm.As the temperature increased to 900 °C (M900 (Fig. 5c)), the particle sizes increased to ∼3.64 mm and the morphology became attened, giving rise to new facets.This condition appears inevitable, primarily because of the interaction between magnetic particles at higher calcination temperatures. 60,61he Brunauer-Emmett-Teller (BET) surface area of the samples was determined using nitrogen adsorption-  desorption isotherms measured at 77.3 K (Fig. 6).From the obtained isotherms, all the samples show type IV adsorption isotherms which indicate mesoporous structures.Furthermore, all samples show H3 hysteresis loop which show the characteristic of slit shape features. 62,63The open loop at the isotherm may be caused by slow adsorption at narrow pores which exhibited from slit shape features. 63The specic surface areas of M700, M800, and M900 were found to be 2.6017, 3.6244, and 7.7535 m 2 g −1 , respectively.In addition, the measured pore volumes of the samples were 0.0020 cm 3 g −1 for M700, 0.0032 cm 3 g −1 for M800 and 0.0082 cm 3 g −1 .It is clear that higher calcination temperatures resulted in an increase in the BET surface area due to structural and morphological changes, indicating the emergence of a new facet as shown in the SEM image. 64,65urther analysis was conducted using TEM images (Fig. 7).The crystalline CoFe 2 O 4 structure demonstrates that the sample M900 possessed dense agglomerates, as illustrated in Fig. 7a.Lattice fringes of CoFe 2 O 4 (Fig. 7b) indicate an interplanar spacing of 0.25 nm belonging to the (311) plane with a cubic phase, which agreed well with the XRD data.
CV was conducted for all electrodes between 0.01 and 3.0 V at a scan rate of 0.1 mV s −1 (Fig. 8).Throughout the rst scan, all electrodes showed a broad-ranging cathodic peak at 0.6 V, consistent with the irreversible emergence of a solid electrolyte interface (SEI); electrolyte deterioration causes a signicant irreversible loss of capacity during the rst discharge process. 33,66The shi between 0.3 and 0.8 V during the subsequent cycle is attributed to the reduction of Fe 3+ and Co 2+ to Fe 0 and Co 0 , respectively, and the reversible reaction to form Na 2 O (eqn (2)): 67,68 CoFe 2 O 4 + 8Na + + 8e − 4 Co + 2Fe + 4Na 2 O (2) In the anodic process, the oxidation peaks at 0.8 and 1.2 V are attributed to the reformation of CoFe 2 O 4 via the oxidation of Fe 0 and Co 0 to Fe 3+ and Co 2+ , respectively. 69,70All the CV curves almost overlapped during the subsequent cycle, indicating high reversibility of the electrochemical reaction. 33,71he selected cycles of the charge and discharge proles for all  electrodes at a current density of 0.1 A g −1 is shown in Fig. 9.The charge and discharge plateau for all electrodes is aligned with the CV peaks.The initial capacity of discharge and charge capacities of the electrode are 617 mA h g −1 (M700), 839 mA h g −1 (M800), and 350 mA h g −1 (M900), respectively.Based on these result, M800 electrode deliver higher discharge capacity due to the uniform morphology, suggesting that large contact interface between electrolyte and electrodes, which lead to high irreversible Na + consumption. 72ontrarily, the large particle size required a longer time for ion transfer into the particles and faces diffusion limitation of Na + within a single large particle. 65,73All electrodes display irreversible capacity loss owing to the formation of the SEI layer and electrolyte degradation during the rst cycle. 74,75owever, there is difference for the second discharge curves of the sample M900 and sample M800 and M700 due to considerable loss in specic capacity of the M900 sample. 76his result agreed well with the capacity value of M900 sample which is lower that M800 and M700.
Fig. 10a demonstrates the cycling behaviour of all electrodes at a current density of 0.1 A g −1 .At the initial cycle, the M800 electrode exhibits the highest discharge capacity (839 mA h g −1 ), followed by the M700 (617 mA h g −1 ) and M900 (350 mA h g −1 ) electrodes.For the M800 electrode, a preserved discharge capacity of 98 mA h g −1 was calculated aer 100 cycles, whereas the discharge capacity for the M700 (76 mA h g −1 ) and M900 (69 mA h g −1 ) electrodes.The reversible discharge capacity of the M800 electrode was 224 mA h g −1 aer the second cycle and continued to decline throughout the 100 cycles, which was possibly due the activation and stabilisation processes within the electrode. 15In  this regard, the observed capacity values of the M800 electrode remain high compared to the other electrodes.Similar trends were also observed for the M700 and M900 electrodes.Aer 100 cycles, the discharge capacities of the M700 and M900 electrodes were 76 mA h g −1 and 69 mA h g −1 , respectively.According to previous reported, [77][78][79] fast capacity fading of materials due to structure collapse and dissolution of materials may occur in electrolyte decomposition.As a result, the improvement in cycling stability of materials is attributed to a delay in structure decay.The specic capacity retained by the M800 electrode aer 100 cycles was 88%, compared to 87% and 80% retained by the M700 and M900 electrodes, respectively.Clearly, the initial coulombic efficiencies were 48%, 33% and 24% for the M700, M800 and M900 electrodes, respectively, owing to uncontrolled SEI layer formations.Aer several cycles, all the electrodes demonstrated high coulombic efficiencies of more than 99% as the SEI layer formation stabilised during cycling. 80The rate capability of all the electrodes was also determined at different current rates, ranging from 0.2 to 1.0 A g −1 (Fig. 10b).The M800 electrode delivered the discharge capacities of 171, 125, 103, 87, 73 and 108 mA h g −1 at the current densities of 0.2, 0.4, 0.6, 0.8, 1.0 and 0.2 A g −1 , respectively.Even though the rate returned to 0.2 A g −1 , the discharge capacity of M800 electrode could still display the maximum reversible capacity, suggesting stable cycling performance.However, the consecutive cycling performances of the M700 and M900 electrodes were unsatisfactory.Aer 66 cycles at various charge and discharge rates, the discharge capacity of the M800 electrode at 0.2 A g −1 remained 108 mA h g −1 , representing approximately 87% capacity recovery.Hence, the improved cycle and rate performance of the M800 electrode is superior to that of the M700 and M900 electrodes, may be due to the well-dened octahedral shape of M800 delivers sufficient active sites for Na-ion, thus reducing the electron and ion transport pathways.
The improved electrochemical performance of CoFe 2 O 4 (M800) could be assigned to the high crystallinity and homogeneous distribution of particles, leading to a high surface area, 81 facilitating electrode-electrolyte interaction and affording increased active sites for electrochemical reactions. 82,83The well-dispersed particles are benecial for excellent performance because they provide a short transport length and a substantial contact area between the active material and electrolyte. 84,85In this regard, the sample must afford a high surface area for promoting the adsorption and  behaviour.This method is well recognised for its costeffective preparation because the products can be produced in large quantities in a short session. 87Scientically, the salt melts during the preparation method owing to the high rate of ion absorption and high ability to dissolve, which can speed up the rate of the reactions. 88To the best of our knowledge, systematic investigations on the parameters inuencing the formation and characteristics of CoFe 2 O 4 during the molten salt method are still lacking.Overall, the discharge capacity of CoFe 2 O 4 discovered in this study is preferable to those previously reported (Table 2).The synthesis of molten salts at low temperatures represents a template and surfactant free, cost-effective, simple and an efficient method for large-scale production.As a result of this study, new insights can be gained for future studies on CoFe 2 O 4 as an anode material for Na-ion batteries.

Conclusions
CoFe 2 O 4 was successfully synthesised using the molten salt method, followed by calcination at 700 °C, 800 °C and 900 °C.The synthesis approach used provides a straightforward and practical way for industrial production.The powder phases, structures, chemical composition and morphology are characterised through XRD, Raman spectroscopy, FTIR, XPS, SEM, BET and TEM.The electrochemical results indicate that the M800 electrode showed excellent performance as an anode material for Na-ion batteries, which could be attributed to the homogeneity, uniform octahedral morphology, and high crystallinity of the material.The M800 electrode revealed a high initial discharge capacity (839 mA h g −1 at 0.1 A g −1 ) and retained the capacity (98 mA h g −1 ) aer 100 cycles.The capacitive retention was ∼88% aer 100 cycles, demonstrating a good rate capability and cycling stability during the insertion and de-insertion of Na-ions.These ndings indicate that this strategy may provide an innovative approach to improving the electrochemical behaviour of CoFe

Fig. 4
Fig. 4 XPS spectra of the survey scan of (a) Co 2p, (b) Fe 2p and (c) O 1s of the CoFe 2 O 4 for M800.

Fig. 7
Fig. 7 TEM images of M900 at (a) low magnification and (b) highresolution TEM image of the CoFe 2 O 4 .
2 O 4 electrodes for use in Na-ion batteries.disordered spinel CoFe 2 O 4 with improved electrochemical performance for sodium-ion batteries and developed conceptualisation.S. U. M., N. H. I., H. M. Y., F. M. D., S. R. M., and L. N. developed the methodologies.The project was supervised by N. H. I.The manuscript has been reviewed and edited by all contributing authors.

Table 1 and
Fig. 1 display all the XRD patterns and Rietveld renement proles of all the samples.All diffraction peaks can be readily indexed as cubic spinel of CoFe 2 O 4 , which agrees well with the conventional CoFe 2 O 4 spinel with the Fd3m space group (JCPDS no.22-1086).

Table 2
Comparative electrochemical performances of CoFe 2 O 4 anode for Na-ion batteries synthesized from various techniques © 2023 The Author(s).Published by the Royal Society of Chemistry RSC Adv., 2023, 13, 34200-34209 | 34207 Paper RSC Advances